In Final Fantasy VII and Final Fantasy VII Remake, Sephiroth refers to the creature known as Jenova as his mother. The circumstances surrounding Sephiroth's birth are considered classified information in the world of FF7, and the secrets of his conception are hidden throughout the lore of the series. So, is Jenova really Sephiroth's mom?
One of the reasons why the Shinra Electric Power Company was able to wipe out all opposition was due to the might of Sephiroth and SOLDIER in FF7. The individual members of SOLDIER were already highly-trained warriors before entering the program. Once they became official members, they were injected with Jenova cells, giving them superhuman strength. Thanks to the Shinra publicity machine, the members of SOLDIER became regarded as heroes, even when they were being used to put down rebellions or crush the nation of Wutai. By far the most famous member of SOLDIER was Sephiroth, whose combat skills were said to be unparalleled, even among his contemporaries.
In Final Fantasy 7, Sephiroth refers to Jenova as his mother. The glimpses of Jenova that the player gets depict it as a Lovecraftian alien monster, so is it really capable of being Sephiroth's mom? The truth regarding Sephiroth's birth has yet to be revealed in the FF7 Remake series, but it was detailed in the Compilation of FF7, and these details will presumably stay similar in Remake's sequels.
[Warning: Potential SPOILERS for future entries in the FF7 Remake series below.]
FF7 Remake's Circumstances Surrounding Sephiroth's Birth
Jenova isn't Sephiroth's biological mother. It's revealed in Final Fantasy 7 that Sephiroth's parents are Hojo and a Shinra scientist named Lucrecia Crescent. When Jenova's body was originally found by Shinra scientists, it was incorrectly identified as a member of the Cetra species. Several attempts were made at integrating Jenova cells into human children (like Angeal and Genesis from Final Fantasy 7's Crisis Core spin-off), but Sephiroth was the only instance where a human fetus was injected with Jenova cells in the womb. This is the reason why Sephiroth is so much stronger than the other members of SOLDIER.
Sephiroth grew up without knowing the truth of his parentage, though he was told that his mother was a woman named Jenova and that she died in childbirth. It wasn't until the Nibelheim incident that Sephiroth learned more about his parentage, as he discovered a tank labeled "Jenova" in the town's reactor. He also discovered the notes left by the scientists in the Shinra Mansion. Sephiroth came to the (incorrect) conclusion that Jenova was a Cetra and that mankind was responsible for the destruction of their people. This prompted Sephiroth's rampage in Nibelheim and his first death within the Nibelheim Reactor before the events of Final Fantasy VII. The only problem is that Sephiroth's alien lineage allowed him to survive within the Lifestream, allowing him to return.